Chapter Three Objectives 1. Define the cell theory and the three major parts of all cells. 2. Describe the chemical composition and functions of the plasma membrane using the fluid mosaic model. 3. Define microvilli, tight junctions, desmosomes, gap junctions, passive transport, active transport, endocytosis, and exocytosis. 4. Describe the structure and function of all cellular organelles. 5. Describe the structure and function of the three parts of the cytoskeleton. 6. Describe cilia, flagella, and parts of the nucleus CELLS: THE LIVING UNITS All organisms are cellular in nature, be they one-celled or multicellular. The individual cells are the structural units of all living things. The human body has trillions of them. Robert Hooke was the first to observe plant cells with a crude microscope in the late 1600's. In the mid-1800's, Schleiden and Schwann proposed that all living things were composed of cells. Then Virchow extended this idea by suggesting that cells arise only from other cells. Since the 1800's, research has provided us with four concepts known as the cell theory: 1. A cell is the basic structural and functional unit of living organisms. 2. The activity of an organism is dependent on both the individual and collective activities of its cells. 3. Principle of Complementarity - biochemical activities of cells are determined and made possible by the specific subcellular structures of cells. 4. Continuity of life has a cellular basis. Perhaps the most striking property of a cell is its complex organization. Cells are amazingly diverse in both size and shape. Cells do have many common structural and functional features. All cells have three major parts: nucleus, cytoplasm, plasma membrane Plasma Membrane: Defines the boundary of the cell and acts as a fragile barrier. The membrane is thin, elastic, and semi-permeable. It is composed chiefly of a double layer, or bilayer of phospholipid molecules with protein molecules dispersed in it. The fluid mosaic model of membrane structure depicts the plasma membrane as:

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1. Sandwich composed of two layers of phospholipid molecules, with the polar head exposed to water inside and outside the cell and the nonpolar tails facing each other and buried in the internal portion of the membrane. 2. Globular proteins are embedded in the bilayer. The amount of protein varies with membrane function and averages 50% of the mass of the membrane. Proteins function as: structural supports, pumps, carriers, receptors, enzymes, ion channels 3. The membrane is a dynamic fluid structure of about the consistency of olive oil. The lipid molecules are free to move laterally. Some of proteins float freely, while others are more restricted. 4. About 10% of the externally facing lipid molecules have attached sugar groups.

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